The structureactivity relationships were as follows:(1) Compound 2 was synthesized to study the substituents in the piperidine ring. Compounds without a substituent (2a) or with a lower alkyl group, eg, methyl (2b), propyl (2c), and allyl (2j), were inactive (Table I). Analogues from butyl (2d) to heptyl (2g) showed potent activity; however, increasing the & to octyl (2h) or decyl (2i) resulted in the loss of the activity. These results suggest that the optimum ...